Android中关于Bitmap的裁剪缩放和创建

Android 中常常需要对图片进行缩放裁剪等处理，这里简单的介绍一下这两种处理方式的方法

1.裁剪

/**
     * Returns an immutable bitmap from the specified subset of the source
     * bitmap. The new bitmap may be the same object as source, or a copy may
     * have been made. It is initialized with the same density and color space
     * as the original bitmap.
     *
     * @param source   The bitmap we are subsetting
     * @param x        The x coordinate of the first pixel in source
     * @param y        The y coordinate of the first pixel in source
     * @param width    The number of pixels in each row
     * @param height   The number of rows
     * @return A copy of a subset of the source bitmap or the source bitmap itself.
     * @throws IllegalArgumentException if the x, y, width, height values are
     *         outside of the dimensions of the source bitmap, or width is <= 0,
     *         or height is <= 0
     */
    public static Bitmap createBitmap(@NonNull Bitmap source, int x, int y, int width, int height) {
        return createBitmap(source, x, y, width, height, null, false);
    }

将图片转换为Bitmap对象，具体如何转换可以到BitmapFactory中查找。将转换了的bitmap对象传入方法中，x为从横轴上开始裁剪的开始位置，y为从纵轴上开始裁剪的开始位置，width为需要裁剪的宽度，height为需要裁剪的高度。然后这个方法返回的就是你裁剪的图片

2.缩放

public Bitmap resizeImage(Bitmap bitmap, int w, int h) {
        Bitmap BitmapOrg = bitmap;
        int width = BitmapOrg.getWidth();
        int height = BitmapOrg.getHeight();
        int newWidth = w;
        int newHeight = h;
 
        float scaleWidth = ((float) newWidth) / width;
        float scaleHeight = ((float) newHeight) / height;
 
        Matrix matrix = new Matrix();
        matrix.postScale(scaleWidth, scaleHeight);
        // if you want to rotate the Bitmap
        // matrix.postRotate(45);
        Bitmap resizedBitmap = Bitmap.createBitmap(BitmapOrg, 0, 0, width,
                height, matrix, true);
        return resizedBitmap;
    }

缩放通过矩阵Matrix来实现，首先获取原始Bitmap的宽高，然后通过你传入的宽高来计算缩放的比率，将比率设置给矩阵，然后通过系统提供的api来实现缩放。